In the commonest prior-art system a stack of sheets, envelopes, or the like is pressed against a so-called suction roller. The roller is rotated in steps and the suction is applied with each step-wise advance.
The principal difficulty with this arrangement is that the roller very frequently picks up more than one sheet. This is due to the fact that the sheets tend to cling together and that the force with which the stack is pressed against the suction roller is excessive. This force also varies as the stack decreases in size.